They sell both versions because they have different target audiences. Think of it like this without being absolute.
Pro is more for individuals or very small companies that only want the core GIS functions and have very small teams or often even just 1 person working on a project.
Desktop is for companies with multiple employees probably working on the same large-scale projects, often from different locations. and may need customizations or even custom-made solutions/programs for example connecting the GIS system to databases to add extra functions and sources of data or web servers for building web apps.
So as you see, even though both versions appear to do the same thing the difference is in how they do it. The desktop allows for remote collaboration, validation, and quality control of workflows (since you assigned multiple people for multiple tasks, you need to keep a certain level of control), Broader database management, and access to the inner engine of ArcGIS through SDK and APIs which actually is one of the things that large companies want/need, and many other things listed on their site.
It's up to you to see what each path offers (I think the link I shared shows that clearly) and also define your own needs and budget. Additionally, keep in mind that no one forbids you from calling/emailing and asking for more info as a "potential customer" and maybe they can clear any confusion you may have about their product.
If I was you tho, I would start with pro (since it's the "smallest" one) and move to higher versions when I master pro or there is a real need (paying customer or applying for a job in big GIS projects)
Hope it helped.
